Agriculture Reference
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digestion in association with earthworms: incubation in faeces results in the excystment
and development of protist populations which are further digested by invertebrates. In a
microcosm experiment‚ five days after earthworms were introduced into sterilised soil‚
soil protist population density was
soil‚ dominantly amoebae (50-60 %)
(Shaw and Pawluk‚ 1986).
The structure of micro-foodwebs has been elucidated for a number of sites (Hendrix
et al.‚ 1986; Elliott et al.‚ 1988; Ingham et al.‚ 1989) and models have been built to
simulate the effects of such assemblages on soil processes (Hunt et al.‚ 1987; de Ruiter
et al.‚ 1993). The characteristics of such foodwebs are largely influenced by the relative
dominance of fungi or bacteria and this‚ in turn‚ is determined by abiotic factors and
management practices. In dry environments‚ most bacteria‚ protists and nematodes are
inactive and soil foodwebs appear to be fungal-based‚ with abundant fungivorous‚
moderately drought-tolerant mites (Whitford‚ 1989). As found in conventionally-tilled‚
temperate-climate agroecosystems in the USA‚ plant residues are distributed throughout
the plowed layer thereby promoting bacterial activities and bacterial based foodwebs.
In 'no-tillage' systems‚ plant residues accumulate on the soil surface‚ promoting fungal
growth and immobilisation of nutrients in the decomposing plant material and the
fungal biomass. Populations of fungivorous faunal groups increase in the surface layers
and decomposition rates are slower (Hendrix et al.‚ 1986).
Distribution and dispersal of microsites
In soil micro-foodwebs‚ interactions occur mainly within the water-filled soil pore space
and the water films that cover solid particles. These microsites are discrete in time
and space‚ and depend largely on soil moisture content and porosity which‚ in turn‚
depend on texture and overall biological activity. Finally‚ the presence of adequate
carbon resources determines the size of the normally large microbial biomass which
forms the nutritive base for the micropredators. The pore size distribution determines
population size at the microsite level. Flagellates and small amoebae colonise pores with
a minimum diameter of 8 the bigger protists and nematodes live in the larger pores
and most nematodes inhabit the external medium. Soil texture and porosity are therefore
critical determinants of micro-foodweb structure (Hassink et al.‚ 1993).
The importance of porosity is demonstrated‚ for example‚ by an experiment in which
the bulk density of humus was increased from 0.25 to 0.41 Mg pores larger than
decreased and the growth of ciliates and thecamaoebae was inhibited which would
reduce predation pressure on the bacteria (Coûteaux‚ 1985). Soil ecosystem engineers
that may largely determine soil structural porosity are thus likely to determine micro-
foodweb communities.
The presence of organic resources and the capacity of the microbiota to move between
soil microsites are critical to the definition of microsites favourable to micropredator
foodweb activities. Such microsites are regularly found in the rhizosphere where root
growth provides energy for the bacteria and their micropredators‚ and disseminates their
communities throughout the volume of soil explored by the root system. Micropredator
foodwebs also occur in other larger systems of regulation organised around the activities
of such soil macro-organisms as earthworms and termites. These are just some of the
many structures which contribute to the overall soil function.
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